1. Field of the Invention
The invention relates to a process for the production of an anode for electrolytic capacitors as well as to anodes produced thus.
2. Description of the Related Art
For a capacitor comprising two metal plates separated by means of a dielectric of a given strength, the capacitance is a function of the dielectric constant of the dielectric, the vacuum dielectricity, the geometric area a well as the distance between the metal plates. In order to arrive at higher capacitance values at a given area, the thickness of the dielectric can be decreased, the effective surface of the metal plates can be increased by roughing and a dielectric with a higher dielectric constant can be used.
It is known that, compared with other metal oxides, aluminum oxide has considerable advantages due to the fact that from an aluminum foil can be generated anodically with electrochemical etching, films with the formation of aluminum oxide with good insulating properties and with high roughness such that very high capacitance values can be attained in spite of the relatively low dielectric constant of aluminum oxide in comparison with other metal oxides, such as those of titanium or tantalum.
GB-B 2 056 503 discloses a process for the production of an electrolytic capacitor in which a flexible substrate is used with a surface onto which an anodizable metal is vapor-deposited. The vapor deposition take place at an angle of incidence of less than 60xc2x0 in the presence of oxygen and at a partial pressure not greater than 10xe2x88x921 Torr to form a porous metal layer. The porous metal layer is subsequently anodized and a capacitor is wound from such foils. The substrate can be aluminum and the anodizable metal can be tantalum or an alloy of two or several metals.
It is the task of the present invention to propose a process for the production of anodes, as well as the anodes themselves with the aid of which, in simple manner, capacitors of higher capacitance can be fabricated.
This task is solved through a process for the production of an anode for electrolytic capacitors from an aluminum foil as the substrate onto which an alloy of aluminum and a further metal whose oxide has a higher dielectric constant than aluminum oxide, is vapor-deposited to increase the surface roughness in several process steps at different angles of incidence in a vacuum with the formation of a porous layer and which is subjected to a subsequent anodic oxidation.
With anodes produced in this way, substantially higher capacitance values at an identical anodization voltage can be obtained than is the case when using conventional aluminum foils.
As an aluminum alloy, one which comprises a valve metal such as titanium, tantalum, niobium, zirconium or the like is preferably used.
In order to attain a surface of the porous layer of maximum size, the aluminum alloy is preferably vapor-deposited with a metal vapor beam in three successive process segments. For structuring a first thin adhering layer with first crystallization nuclei, in a first process segment the angle of incidence between the metal vapor beam and a surface normal should be large, for example at least 75xc2x0. In order to make optimum growth of crystals with dendritic structure possible, in a second process segment, medium angles of incidence, for example between 40xc2x0 and 60xc2x0, are provided. At the conclusion of the crystal growth, in a third process segment, the aluminum alloy is vapor-deposited at any desired angle, preferably however at a small angle of incidence, for example maximally 40xc2x0.
In one process segment, the vapor deposition advantageously takes place in several process steps at differing angles of incidence. It is, in particular, meaningful to provide in the plane of incidence angles on both sides of the surface normal in order to attain growing-on of the crystals symmetrical with respect to vertical.
To improve the stability of the porous layer, the aluminum alloy can be vapor-deposited in one or several process steps while a protective gas such as, for example nitrogen, is introduced. This is, in particular, advantageous during the last process segment for the stabilization of the crystalline structure.
Depending on the desired results, the aluminum alloy is advantageously vapor-deposited at a foil temperature between 50xc2x0 C. and 300xc2x0 C.
A further increase of the capacitance can he attained if, after the vapor deposition and oxidation, the aluminum foil is subjected to a heat treatment for a given length of time of, for example, approximately 1 to 3 hours, for example in air or an inert gas atmosphere at a given elevated temperature of, for example 350 to 500xc2x0 C. For example, argon can serve as the inert gas.
It is further proposed with the invention to subject the aluminum foil after the temperature treatment to a repeated oxidation.
As the substrate, for example, an aluminum foil with a percentage purity of approximately 99.5 is suitable.
Good values for the capacitances were attained in particular if the aluminum alloy comprises a fraction of valve metal between approximately 20 and 40%.
To heat and vaporize alloy metals disposed in one or several melting crucibles, for example, an electron beam is used with an energy adaptable appropriately to the particular metals. In this way, a uniform deposition of the alloy on the aluminum foil is obtained.
The invention relates also to anodes which have been produced according to the previously discussed process, as well as to electrolytic capacitors with such anodes.
The following Table shows the results of comparison experiments for aluminum foils of conventional type and aluminum foils according to the invention with Alxe2x80x94Ti or Alxe2x80x94Ta alloy coating with the alloys being anodized at constant growth rate under identical experimental conditions and at the identical final voltage Vf=22 V. It can be seen that higher capacitance values for aluminum foils with Alxe2x80x94Ta alloy coating or Alxe2x80x94Ti alloy coatings were attained, wherein the tantalum content was approximately 35%, and the titanium content was approximately 20 to 40%. Evident are also the still higher capacitance valves measured on samples after the heat treatment. To some extent, a doubling of the capacitance values resulted.
Physicochemical examination of the dielectricity films by means of photocurrent and electrochemical impedance spectrometry showed that the mixed oxide films behave like insulators with relatively large band intervals and very low dielectricity losses. The large band interval values, low dielectric losses in a wide frequency range (1 to 1.5 kHz) and better chemical stability of the mixed oxides in aqueous solutions, in comparison to an aluminum oxide, make the use of the films as dielectrica for electrolytic capacitors especially suitable.